Liquid immersion and circulation cooling systems of the type disclosed in U.S. Pat. No. 4,590,538 by Seymour R. Cray, and assigned to Cray Research, Inc., have been successfully applied to the design of supercomputers. In such systems, the logic and memory circuits are assembled in high-density modules arranged in a plurality of vertical stacks positioned closely adjacent one another in a tank or container filled with inert cooling liquid. The adjacent stacks of circuit modules are spaced apart to form coolant flow columns therebetween, and alternate ones of the coolant flow columns are used for supplying coolant into the container and alternate ones are used for removing coolant from the container in such a fashion that coolant flow is established across and between the circuit modules in the stacks. The removed coolant and some vapor bubbles formed therein through heating are processed through a heat exchanger external to the container holding the computer and reintroduced. A reservoir is also connected to the container for holding the coolant during pump-down to service the computer. This technique has proven effective for successfully cooling a four-processor supercomputer having 256 million words of random access memory, all fitting within a container of approximately 28 cubic feet and dissipating approximately 160,000 watts of power.
The inert liquid used is a fluorocarbon product called Fluorinert, made by the 3M Company. The circulation rate of the inert liquid through the computer is chosen in consideration of the heat load produced by the computer and the characteristics of the liquid. It is normal that some vapor bubbles will be formed and rise to the top, where they are carried down with the outlet flow of the coolant to eventually recondense. However, substantial boiling is to be avoided, as excessive production of vapor bubbles greatly decreases cooling efficiency.
Generally, in such immersion-cooled supercomputers, the total power consumption and heat dissipation are so great that the temperature within the computer could get rapidly out of control if the circulation system were impaired, for example, by a failure of a pump or heat exchanger. For that reason, monitoring systems have been used which monitor components of the cooling system and which include coolant temperature and flow sensors. Therefore, if there is a temperature rise due to a system failure, it will be quickly detected and the computer can be shut down to prevent damage. This is especially important not only to protect the computer hardware from temperature-induced damage, but also because it is recognized that the fluorocarbon inert liquid coolants can break down into toxic gases at high temperatures above approximately 200.degree. C., and these toxic gases could pose a hazard to personnel in the area.
The breakdown temperature is so much higher than normal operating temperature that it is unlikely that breakdown would ever occur. Nevertheless, to guard against this possibility, ventilation systems have been installed which operate, when activated, to remove vapor from inside the coolant reservoir and exhaust it in a safe location outside the building away from personnel. Fresh air is drawn into the coolant reservoir to replace the vapor thus removed. The toxic gases, though toxic in high concentrations in a confined area, do not present a hazard when exhausted through a suitably located outdoor exhaust pipe, which reduces the concentration of the gas to levels which are not harmful. The system was designed to be activated by a computer operator upon observation of excessive bubbling within the computer container. For greater reliability, it was proposed to make the activation of the ventilation system automatic, but detection of bubbles in the cooling system is not a reliable method for activating the exhaust system since a certain amount of bubbles are normally formed in the cooling process. Direct detection of the gases produced by breakdown of the fluorocarbon through physical or chemical means is not practical.
In accordance with this invention, it is proposed to indirectly sense for breakdown of the fluorocarbon coolant by precise monitoring of the power supply voltages to the computer. This is based upon the reasoning that temperatures high enough to cause breakdown can only be produced in two ways within the computer tank. One is through partial or total failure of the circulation system, which will produce a relatively rapid rise in temperature in the coolant within the tank, or in the coolant within one or more stacks of the computer associated with an impaired or failed coolant supply. Such temperature rise, although very rapid, can be detected by the above-mentioned coolant temperature and flow, and cooling system component monitoring, and automatic controls can shut down the computer before excessive temperatures are reached so the breakdown would be avoided.
The other way in which coolant breakdown could occur is a local "hot spot" within the container which would not be immediately detected by the above-mentioned monitoring system. Such a "hot spot" would have to be caused by electric arcing due to a catastrophic electronic failure or short circuit, such as foreign matter or a loose part lodging between conductors carrying power and ground potentials. A fault of this type could produce arcing that would result in high temperature at the immediate vicinity of the arc, which could produce coolant breakdown. The toxic gas would then rise as bubbles to the top of the container and would probably be drawn off to the reservoir through normal reservoir circulation. Any such short circuiting or arcing would result in a detectable variance in voltage on the bus or buses in question.